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[Attention] Make CutlassMLA the default backend for SM100 (blackwell) (#21626)

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Signed-off-by: Alexander Matveev <amatveev@redhat.com>
Signed-off-by: mgoin <mgoin64@gmail.com>
Co-authored-by: mgoin <mgoin64@gmail.com>
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Alexander Matveev 2025-07-27 16:13:00 -04:00 committed by GitHub
parent a9b2a1d704
commit 8f605ee309
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1 changed files with 22 additions and 7 deletions
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29
vllm/platforms/cuda.py
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@ -150,11 +150,28 @@ class CudaPlatformBase(Platform):
# TODO(lucas): handle this more gracefully
# Note: model_config may be None during testing
if model_config is not None and model_config.use_mla:
# if `VLLM_ATTENTION_BACKEND` is not set and we are using MLA, then
# we default to FlashMLA backend, so we need to force the blocksize
# here
use_flashmla = (envs.VLLM_ATTENTION_BACKEND is None \
or envs.VLLM_ATTENTION_BACKEND == "FLASHMLA")
# If `VLLM_ATTENTION_BACKEND` is not set and we are using MLA,
# then we default to FlashMLA backend for non-blackwell GPUs,
# else we default to CutlassMLA. For each case, we force the
# required block_size.
use_flashmla = False
use_cutlass_mla = False
if envs.VLLM_ATTENTION_BACKEND is None:
# Default case
if cls.is_device_capability(100):
# Blackwell => Force CutlassMLA.
use_cutlass_mla = True
envs.VLLM_ATTENTION_BACKEND = "CUTLASS_MLA_VLLM_V1"
else:
# Not Blackwell
use_flashmla = True
else:
# Forced case
use_flashmla = (envs.VLLM_ATTENTION_BACKEND == "FLASHMLA")
use_cutlass_mla = (
envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA_VLLM_V1")
from vllm.attention.ops.flashmla import is_flashmla_supported
if use_flashmla and is_flashmla_supported()[0] \
and cache_config.block_size != 64:
@ -162,8 +179,6 @@ class CudaPlatformBase(Platform):
logger.info(
"Forcing kv cache block size to 64 for FlashMLA backend.")
use_cutlass_mla = (envs.VLLM_ATTENTION_BACKEND is not None \
and envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA_VLLM_V1")
if use_cutlass_mla and cache_config.block_size != 128:
cache_config.block_size = 128
logger.info("Forcing kv cache block size to 128 for "